The present invention relates generally to display elements. More particularly, the present invention relates to display elements that include a network and a mobile phase, which under the influence of an externally applied electric field, displaces from one location to another through or within the network to form part of an image or alternatively, to serve as a light valve for optical switching applications.
Electrophoretic displays enjoy significant advantages over the alternatives of cathode ray tubes (CRTs) and liquid crystal displays (LCDs) particularly for portable display applications. Specifically, electrophoretic displays require significantly less power than the bulky CRTs and provide a wider field of view, provide significantly less undesirable light absorption, and are manufactured at a lower cost than the LCDs. Significantly, electrophoretic displays enable the production of highly transmissive, highly reflective, or “paperlike,” displays, which are not achievable with current technologies. For more information on the advantages of electrophoretic displays, reference may be made to U.S. Pat. No. 5,582,700 issued to Bryning et al., which is incorporated herein by reference in its entirety for all purposes.
FIG. 1 shows a prior art display element 10 generally described in U.S. Pat. No. 4,419,663 issued to Kohashi. Display element 10 includes two electrodes 12 and 14, which are shaped like plates and spaced apart to define a space therebetween. Typically, at least one of electrode 12 is transparent, allowing a viewer 20 to view display element 10. Disposed in a portion of the space between electrodes 12 and 14 is ink 16, which is a light transmissive liquid material and impregnated in numerous distinct pores or micropores found in a porous material 18. In this configuration, there exists a gap between electrodes 12 and porous material 18. Ink 16 can be transparent and porous material 18 can be white. Furthermore, the index of refraction of ink 16 and porous material 18 are substantially equal. As a result, when numerous pores inside porous material 18 are completely or almost completely impregnated with ink 16, external light entering through the transparent electrode is not reflected at the contact interface of ink 16 and porous material 18, but is transmitted therethrough to provide a display element which appears to be transparent.
During a typical operation, under the influence of a voltage potential applied by a voltage source 22 through leads 24 and 26 to electrodes 12 and 14, ink 16 electroosmotically moves out of the pores of porous material 18 and moves towards the negatively charged electrode. During such movement, excess ink 16 occupies the gap between electrode 12 and porous material 18. Consequently, voids filled with air exist inside the pores of porous material 18. The presence of air contributes to a mismatch in the index of refraction between porous material 18 and voids inside the pores. As a result, external light, which enters through transparent electrode 12, is reflected to provide a display element which appears to be white. When the electric field is reversed, ink 16 returns to porous material 18 by capillary action, impregnating the porous material.
If a black background is placed underneath the display element, when it is in its transparent state, light entering the display element is absorbed by the black background, and not reflected. The display element with the black background appears black when viewed from the direction of the incident light. With the same black background in place, but when the display element is in its white state, the light entering the display element is reflected from the porous material, and the display element will appear white.
By way of example, FIG. 2 shows an image of a cross (i.e., “+”) 50 formed by an electrophoretic display. To form image of a cross 50, certain display elements 52 appear colored black (hereinafter “black elements 52”) and certain other display elements 54 contrastingly appear white (hereinafter “white elements 54”). According to the example of FIG. 1 described above, some display elements under the influence of an electric field will appear black (when light is absorbed by the black background), while other display element, at the same time under reversed electric field will appear white (when the porous material is of white color). A combination of these numerous black elements 52 and numerous white display elements 54 together form image of cross 50 shown in FIG. 2.
Unfortunately the described display element found in the prior art suffer from several drawbacks. For example, it takes the ink a relatively long time to return from outside the porous material back into the pores of a porous material through capillary action. As a result, the prior art display elements suffer from poor switching speed. As another example, electroosmotic movement of the ink requires expending significant amount of energy, raising the power requirements for this design of display element. Furthermore, the design is complicated, requiring a gas-containing gap within the display element.
What is, therefore, needed is an improved display element that effectively facilitates the formation of an image, without suffering from the drawbacks, e.g., poor switching speed and high power requirements, encountered by the prior art display elements.